1. Field of the Invention
The present invention is directed to a fire detector unit, and more particularly to a fire detector unit of a scattering light detection type.
2. Description of the Prior Art
Fire detector units of a scattering light detection type have been widely utilized to monitor a smoke density which is proportional to an amount of light scattering due to the presence of smoke particles, and to determine the fire presence by comparing the smoke density with a predetermined threshold. Japanese Utility Model Publication No. 4-108293 discloses one typical fire detector unit which includes a base with a smoke chamber, a light emitting diode directing an incident light into the smoke chamber, and a photo diode collecting a light scattering due to the smoke particles in the smoke chamber to generate an electric signal indicative of the amount of the light received. The electric signal is processed at a fire detecting circuit which provides a fire warning signal when the detected smoke density becomes critical. Included in the detector unit is a circuit board which is secured to the base and mounts the light emitting diode, the photo diode, and electronic components forming the fire detecting circuit. In order to make an electromagnetic shield over a particular portion of the electric circuitry for protection against a possible radiation noise, the detector unit is provided with a metal-made shield which is formed separately from the base and is assembled together with the circuit board on the base. Since the shield is formed separately from the base, an extra work is required to apply the shield to the circuit board and fix the shield to the base, in addition to mounting the circuit board to the base, when assembling the detector unit, thereby lowering manufacturing efficiency. Therefore, it is not easy to assemble the detector unit at a low manufacturing cost with the use of an automatic fabrication technique.
In view of the above insufficiency, the present invention has been achieved to provide an improved fire detector unit which is capable of being fabricated efficiently at a low cost, yet assuring a desired electromagnetic shield over a portion of an electric circuitry inherent to the detector unit. The fire detector unit in accordance with the present invention includes a base made of a molded plastic to have a labyrinth wall which projects on the circumference of the base to define therein a smoke chamber. The labyrinth wall permits an entry of smoke particles but prohibits the entry of an ambient light into the smoke chamber. The base carries a light projector which directs an incident light from a light emitting element into the smoke chamber. The base also carries a light collector which collects a light scattered by the smoke particles in the smoke chamber to a light receiving element. The light receiving element generates an electric signal indicative of the amount of the light received. A fire detecting circuit is connected to receive the electric signal so as to provide a fire warning signal based upon the electric signal. The light emitting element, the light receiving element, and the electronic components forming the fire detecting circuit are mounted on a circuit board which is assembled on the base. Included in the detector unit is a metal-made electromagnetic shield which protects the light receiving element from electromagnetic radiation noises. The characterizing feature of the present invention resides in that the electromagnetic shield is integrally molded into the base and has a ground terminal for connection with a ground line of the circuit board, and that the circuit board is fixed to the base by means of metal-made terminal pins which are also integrally molded in the base for electrical connection with the fire detecting circuit and which project through the circuit board for connection with an external line. With the provision of the molded-in electromagnetic shield and the molded-in terminal pins, the electrical connection of the shield to the electric circuitry as well as the connection of the circuit board to the base can be made simultaneously simply by mounting the circuit board to the base, thereby facilitating the assembly of the detector unit.
In a preferred embodiment, the labyrinth wall is molded together with the base to form a unitary structure in which the electromagnetic shield is embedded, thereby reducing the number of the parts for easy assembly of the detector unit.
Preferably, the electromagnetic shield and the terminal pins are prepared from a single metal sheet by striking the metal sheet and bending the struck portions thereof, which also makes it easy to fabricate the detector unit.
The detector unit may further include an insect deterring cover which is molded from a plastic material to have a side wall and a bottom wall. The side wall is in the form of a screen which surrounds the labyrinth wall so as to prevent flying insects or the like foreign matters from entering the smoke chamber, and has a number of air vents permitting the entry of the smoke particles into the smoke chamber through the labyrinth wall. The bottom wall is provided for covering an open bottom of the base to close the smoke chamber. Formed on the interior surface of the bottom wall are first and second masks which make the light projector intact from a light not coming directly from the light emitting element and make the light collector intact from a scattered light not due to the presence of the smoke particles. Thus, the light reflected from the interior surface of the bottom wall can be successfully excluded from the smoke density detection to enhance the reliability of the smoke density detection.
Preferably, the side wall of the insect deterring cover is formed with at least one blind section devoid of the air vents. The blind section extends over a limited circumference of the labyrinth wall in an immediately opposed relation to one of the light projector and the light collector. The air vents on opposite of the blind section are so oriented as to direct the ambient air towards the labyrinth wall along a direction generally parallel to a line connecting the blind section to a geometric center of the base. Thus, the air on opposite of the blind section can be guided smoothly into the smoke chamber to compensate for deficiency of the air flow that is prevented from entering the smoke chamber by the presence of the blind section, which is necessary for avoiding any inadmissible light leak through around the light protector and the light collector.
The labyrinth wall includes a plurality of L-shaped studs each having an outside corner and an inside corner. The L-shaped studs are arranged circumferentially around the base in such a manner that the outside corner of the L-shaped stud projects into the inside corner of the adjacent L-shaped stud. Two of the L-shaped studs disposed forwardly of the light collector along an optical axis of the light collector are joined at the outside corners to form thereat a combined stud of a generally X-shaped configuration. The X-shaped combined stud is found advantageous to the entry of the ambient light into the field of view of the light collector, while minimizing the loss of the air flow into the smoke chamber.
The X-shaped combined stud is formed with a V-shaped recess which opposes to the light collector with respect to the optical axis thereof and constitutes a light trap responsible for preventing the incident light from reflecting towards the light collector.
The light trap in the form of the V-shaped recess is defined by a pair of first and second legs each being a part of the combined stud with the first leg located closer towards the light projector than the second leg. The light trap also includes a shield ledge which projects from the first leg and a concave at the bottom of the recess. The concave is hidden behind the shield ledge from the light projector. Thus, the light from the light projector as well as the light reflected from other portions of the smoke chamber can be successfully prevented from being reflected towards the light collector, thereby minimizing undesired stray light.
In order to further enhance the capability of minimizing the stray light, the light trap may be further provided with a reflecting section in the form of a serration which is opposed to the light collector and is configured to reflect the incident light deep into the V-shaped recess away from the light collector.
Further, the base is preferred to include a shielding post which projects at a location between the light trap and the light projector in a spaced relation respectively therefrom for interruption of the light from the light projector towards the light trap. The shielding post is also located outside of an incident angle of the light collector. Thus, the light collector is well protected from receiving the light not due to the presence of the smoke particles for increased detection reliability.
The light emitting element and the light receiving element are mounted on the circuit board so that, when the circuit board is secured to the base, the individual optical axes of these elements extend generally perpendicular to a plane of the base. In this connection, the light projector has a light projecting axis which extends within the smoke chamber in parallel with the plane of the base, and the light collector has a light collecting axis which extends within the smoke chamber in parallel with the plane of the base in a crossing relation with the light projecting axis. The light projector forms a first light guide which changes the direction of the light beam from the light emitting element to direct it along the light projecting axis. Likewise, the light collector forms a second light guide which changes the direction of the light collected along the collecting optical axis to direct it along the optical axis of the light receiving element. With this architecture, a light emitting diode (LED) utilized as the light emitting element can be mounted upright on the circuit board without being accompanied with an otherwise necessary awkward work of bending the leads of LED. Further, the upright mounting of LED can minimize the length of the leads and therefore an overall height dimension of the assembly of the base and the circuit board, contributing to give a low-profile structure of the detector unit.
Preferably, the light projector and the light collector are each in the form of an optical prism. The optical prism defining the light collector may includes an integrally formed converging lens which converges the collected light towards the light receiving element for improving detecting efficiency.
These and still other objects and advantageous features of the present invention will become more apparent from the following description of the preferred embodiments when taken in conjunction with the attached drawings.